Protective treatment for mg and mg-based alloys



3,183,125 PROTECTIVE TREATMENT FOR Mg AND M'g-BASED ALLOYS Carli Kenneth Bjork, Midland, Mich, assignor to The DowChemical Company, Midland, Micln, a corporation. of Delaware No Drawinga Filed Mar. 2, 1961, Ser. No. 92,760

4-Claims. (Cl. 148-62) Theinvention relates to a method of applying a protectivercoating to magnesium and magnesium-base alloys and more particularly relates to an improvement in the method of treating particulate magnesium or magnesium-base alloys withan aqueous alkali metal dichromate solution.

For the purposes of the specification and claims magnesium and magnesium-base alloys containing at least 70 percent by weight of magnesium are hereinafter referred to as magnesium metal.

Particulate magnesium metal, during storage as the powder or pellets, tends to react with atmospheric moisture' and oxygen and thereby acquires an oxide coating. The coating tends to make the metal less effective in subsequent? use in incendiary or explosive compositions and the like. Also, on compounding and then storing such compositions, the metal may become less reactive, or'the metal may react with components in the composition whereby the metal is largely consumed before use.

Prior attempts have accordingly been made to provide a protective coating for particulate magnesium metal which does not markedly decrease the reactivity of the metal duringrapid combustion or explosive reactions. Heretofore, one of the best ways of protecting particulate magnesium metal has been to form a coating on the particles by subjecting the particulate metal to boiling aqueous dichromate solution for a brief period such as 5-15 minutes whereby a thinprotective dichromate conversion coating is formed on the surface of substantially all the particles or pellets. The dichromate coating, perhaps because of its bound oxygen content, has not greatly decreased the reactivity of the metal while providing protection from moisture.

It has now been found that the corrosion resistance of dichromate treated particulate magnesium metal' may be further greatly improved, without substantially adversely affecting the reactivity of the metal to rapid combustion or explosive reaction, by the application, to the dichromate. treated metal, of a hydrophilic-hydrophobic surface active. material having sufi'icient atfinity for the dichromated surface of the metal whereby the surface is made water'repellant.

Any magnesium metal may be treated according to the process of the invention, e.g., magnesium, and the magnesium-base alloys having the ASTM or ASTM type designations AZ31A, AZSIB, HK31,.ZK60, HM21, ZHl l, ZEA, and the like. Particulate metal having particle sizes up to 0.1 inch diameter 'or more are advantageously treated according to the invention.

\ Surface active materials which may be employedare those which are capable of depositing an adherent water repellant covering or layer'over the dichromate coating States to on the magnesium particles. The requisite combination of properties whereby a material is adherent to a chromated magnesium surface, yet is water repellant is found in ahydrophobic, water disperable, oxygen-contaim ing, surface active compound containing an uninterrupted ice those that are readily mechanically emulsified in water as well as those that are water soluble.

An uninterrupted carbon to carbon chain containing from 8 to 26 carbon atoms is a sequence of 8 to 26 carbon atoms connected in a series unbroken by intervening non-carbon atoms such as oxygen or nitrogen. The bonding in the sequence may include olefinic bonds, as in the decylenic or linolenic groups, as well as single, or saturated, bonds, as in the lauric or stearic groups.

A carbonoxygen polar group linkage is one having a carbon to oxygen bond, and a substituent atom or group i .on the oxygen or the carbon whereby the polar group exhibits an asymmetrical displacement of charges, or a dipole effect within the group. The charge separation or the dipole effect must provide sufficient polarity to pro-' vide an afiinity for the highly ionic chromated surface of the metal so that the polar group is closely held thereto. Under these conditions water, or aqueous solution, does not readily break the bond between the surface active material and the chromated surface. The carbon oxygen polar group linkages include:

on ii H I Groups of compounds which exhibit the desired combination of properties have a long carbon chain group, as indicated above, to impart hydrophobic character and at least one of carboxy, amino, amido or polyhydroxy groups to impart hydrophylic character. Specific examples of groups of compounds are: alkali metal salts of long chain saturated and unsaturated fatty acids having about 8 to 26 carbon atoms, ethylene oxide condensates of fatty acid amides, amine fatty acid condensates, and alkylphenoxypolyalkoxyalcohols.

For ease of application of the surface active material it is' preferred that the material be water dispersable. The material can then be conveniently admixed with, or otherwise incorporated in, the wash water used in the process of dichromate treating the particulate magnesium, though the surface active material may be applied, to the particulate magnesium, if desired, as a solution. or dispersion in an organic solvent, e.g., ethanol.

In providing chromated particulate magnesium metal, aground or atomized form of particulate magnesium metal is treated with hot aqueous alkali metal dichromate solution, as well understood in the art, e.g., by treating the metal with from about 5 to 20 cc. of a 10 to 20 percent by weight dichromate solution per gram of metal at about the boiling temperature of the dichromate solution. The metal is usually poured into the hot dichromate solution and the mixture is stirred occasionally during a 10 or 15 minute period. Then the metal is separated from the aqueous solution as by suction filtration.

In carrying out the processof the invention, the dichromate treated metal is washed with an aqueous solutionof water and 0.05 to 0.5 percent by weight or more, of asurface active agent of the type defined hereinabove. If the dichromate treated metal has been separated from the dichromate solution by filtration, it is convenient to wash the metal on the filter. About 5 to 10 cc. of the 3. solution of surface active agent per gram of metal provides an adequate treatment. a

After the wash solution has filtered or otherwise drained'away from the metal, the metal is dried. Air

therethrough. The tubing was connected to an open end mercury filled manometer. The flask was shaken causing the boat to tip and bring the magnesium powder into contact with the acid solution. The flask was then imdrying may be employed if desired, but oven drying, as mersed in a constant temperature bath maintained at 25 at 105115 C., is to be preferred as quicker and more C. The increase in manometric pressure was read after positive in assuring spreading of the surface active maa 15 minute interval. The results of this test and the terial over the surface of the particle and in enhancing surface active agents employed are listed in the table. the adherence of the surface active material to the par- In a manner similar to that described in the foreticulate metal. going examples the following surface active compounds The beneficial effect of the surface active agent is not or mixtures thereof are found to adhere to dichromate entirely understood but it is believed that it is deposited treated particulate magnesium and to impart water repelon the dichromated metal as a water repellant layer lancy thereto: which seals the pores of the original coating (formed Tradename: Type of compound during dichromate treatment) and reduces the wettabiltty 53 Polyoxyethylene stearate. of the particulated metal Tween Polyoxyethylene sorbitan Examples monolaurate.

The following examples serve only to illustrate th i gfl z g i i g' advantageous results obtained on following the practice 20 max fi es 5 m Xe of the invention and the scope of the invention is not to y an i be considered limited thereby S1pex TEA Triethanolamine salt of lauryl Portions of ground magnesium powder passing a No. A] inc 204 s f i F f es 325 sieve (U. S. Sieve Series), weighing 5 grams, were ggi S ii i i ig g each placed in a 50 cc. quantity of boiling aqueous sodi- 3121i m y W um dichromate solution (15 grams bla Cr O -2H O/lO0 Avasol Fatty acid derivative' cc. H 0) for ten minutes. Each dichromated portion Cathol Fatty amido diamme. was separated from the dtchromate solution by suction C Hc Fan mide d riv tive filtration and washed with a dilute aqueous solution of a g i6"" T y e t water soluble surface active agent, and oven dried at 105- t omeen er p e y ene e 0 C condensation products of the By way of comparison. an additional 5 gram portion of P h T s pnswry fatty ammes' ground magnesium powder passing a No. 325 sieve was omecn similarly treated with boiling dichromate solution, scpa- Ethomeen rated from the dichromate solution, washed with dis- Eihomfien 18/15 mk water, and oven dried Ethomid C/lS Ethylene oxide condensates of Each of the dichromate treated portions, including fatty acld amidesthe comparison portion, was tested to determine the rela- Ethomld RO/ 15 P- tive resistance of the treated powder to corrosive attack Eureka catlqmqderlvatlve of a fatty by each of 10 percent by weight and 15 percent by weight acid dlamme Cond6n5aieaqueous perchloric acid solutions. The extent of relgepalcA-630 Octyl phenoxypoly(ethyleneaction, i.e., corrosion, of each portion was determined oxwethanolmanometrically as follows: lgepal CA-730 Do.

Treated magnesium powder (01500:.0007 gram) 4 Igepal CA-2l0 Nonyl phenoxypoly(ethylenewas accurately weighed into a plastic boat. cc. of oxy)ethanol. perchloric acid solution were pipetted into a 500 cc. Janusol Mixture of lauryl and myristyl Erlenmeyer flask. The boat was carefully inserted in the esters containing both priflask so that it floated on the surface of the acid solution mary amino and sulfated and the powder did not contact the acid solution. The groups. flask was closed by a stopper having tubing extending Liquex 11 Fatty acid alkanolamide.

Surface active material Extent of corrosion in l5m1nutcs Sample N o. Concen- Aqueous Concen- Remarks tration in N B40104, tration, 15%; Identity wash solu- 10%; prespressure tion, percent sure change, change,

by weight mm. g mm. Hg

1 Intracol 0A, (3.8 0. 5 0 0 Powder vetted very little. 2 d0 0.25 5 0 Pcivfiv ger l gated slightly by 10% soln., very little by S 0. 8 12 Powd er wetted somewhat. 0. 0 Powder wetted very little. 0. 15 Powder wetted somewhat. 0. 0 9 P%wd1e5r 7wetlted very little by 10% soln., Somewhat SO D. 0. 3 Poviider vetted very little.

. 2 22 Powder wetted very little by 10%soln., some wetting by 15% soln. Comparison A 26 54 Chromated powder readily wetted by soln. Comparison B Plaini magnesium powder reacted vigorously with l Distilled water.

' Rapid pressure build up too large to measure in apparatus within 1-2 minutes.

Intraeol 0A, (J.S.=lntrar-.ol 0A cationic surfactant, an acid salt of a long chain fatty acid amide having multiple amine groups on chain. Aerosol C-fil, (LS. =At-rosol (1fi cationic surfactant, ethanolatetl nlkyl guar'tidino-amine complex, Arquud S, (..S.:=Arquad S cationic surfactant, n-alkyl trimethylammonium chlorides.

Comparison A=(. l|rmuat -.d magnesium powder. Comparison B Iltllll magnesium powder.

Tradename: Type of compound Michelene Diamine fatty acid condensate.

Michelene l5 Do.

Michelene 30 Amine fatty acid condensate.

Michelene 45 Do.

Michelene D-4- Sulfa-ted fatty acid amide.

Muslor K Blend of fatty acid esters and polyglycols with complex amino bases.

Onamine R0 High-molecular weight tertiary amine.

Onyx-o1 12' Mixed fatty acid monoethanolamine condensate.

Onyx-o1 336 Laurie acid diethanolamine I condensate.

Sapamine PA Carboxylic acid amide derivative.

Solar NP liquid concentrate Ethoxylated nonyl phenol. Unicote Fatty nitrogen complex. Uniwax Fatty amide complex. Wilimid T Polyalkylene amine.

The invention having been thus described, other modifications will be obvious to those skilled in the art, and

v.the scope of the invention is to be considered limited only by the scope of the appended claims.

I claim: 1. In a method for providing a corrosion resistantcoating on particulate magnesium metal which includes contacting said metal with a hot alkali metal dichromate solution, the improvement which comprises: washing the resulting chromated particulate magnesium metal with a dilute dispersion containing at least 0.05 percent by urn metal.

3. In a method for providing a corrosion resistant coating on particulate magnesium metal which includes contacting said metal with a hot alkali metal dichromate solution, the improvement which comprises: washing the resulting chromated magnesium metal with a dilute dispersion containing from 0.05 to 0.5 percent by weight of an acid salt of a long chain fatty acid amide having as substituents on said long chain at least two amine groups.

4. In a method for providing a corrosion resistant coating on particulate magnesium metal which includes contacting said metal with a hot alkali metal dichromate solution, the improvement which comprises: washing the resulting chromated magnesium metal with a dilute dispersion containing from 0.05 to 0.5 percent by weight of an ethanolated alkyl guanidine-amine complex.

References Cited by the Examiner UNITED STATES PATENTS RICHARD D. NEVIUS, Primary Examiner.

RAY K. WINDHAM, Examiner. 

1. IN A METHOD FOR PROVIDING A CORROSION RESISTANT COATING ON PARTICULATE MAGESIUM METAL WHICH INCLUDES CONTACTING SAID METAL WITH A HOT ALKALI METAL DICHROMATE SOLUTION, THE IMPROVEMENT WHICH COMPRISES: WASHING THE RESULTING CHROMATED PARTICULATE MAGNESIUM METAL WITH A DILUTE DISPERSION CONTAINING AT LEAST 0.05 PERCENT BY WEIGHT OF A HYDROPHOBIC, WATER-DISPERSABLE, OXYGENCONTAINING SURFACE ACTIVE AGENT, SAID AGENT HAVING AT LEAST ONE UNINTERRUPTED CARBON TO CARBON CHAIN, SAID CHAIN CONTAINING FROM 8 TO 26 CARBON ATOMS, AND SAID AGENT CONTAINING AT LEAST ONE CARBON OXYGEN POLAR GROUP LINKAGE, SAID LINKAGE HAVING SUFFICIENT POLARITY TO PROVIDE AN AFFINITY FOR THE CHROMATED SURFACE OF SAID CHROMATED MAGNESIUM. 